<!DOCTYPE html>
<html>
  <style>
    #header {
      position: fixed;
      top: 0;
    }
    #header > div {
      border: solid blue 2px;
      height: 100px;
      width: 100px;
    }
  </style>
  <div id="section">
      <div id="header">
        <div></div>
      </div>
  </div>
</html>